This invention is directed to laser surface treating and, in particular, to apparatus for coupling the laser optical energy to the surface.
Laser surface treating is an industrial process by which the surface of a metal or other material is rapidly heated by a laser beam. The subsequent quenching by heat conduction when the laser is turned off produces a hardened surface layer in metals which is better defined than the layers obtained by flame or induction hardening. Various materials can also be predeposited on the surface in order to obtain wear or corrosion resistant surface alloys. Laser processing is potentially a very efficient technique because it can treat preferentially a specific portion of the surface, and very little energy is lost through heating of the bulk material.
One of the problems faced by this technique is the small coupling coefficient of the laser radiation into surface heat. Most metals have reflectivities of the order of 90% or more, particularly at the long IR wavelength of the industrial CO.sub.2 lasers. Semiconductors such as silicon are also highly reflective in the molten state. Consequently, a very small fraction of the laser energy is absorbed by the metal's surface. Absorbing paints are used to increase the absorption coefficient, but this technique is not satisfactory for a number of reasons. The paint is quickly vaporized by the laser beam, leaving the bare metal's surface for most of the laser's dwell time. Part of the laser's energy must be spent against the heat of vaporization of the paint, thereby further reducing the overall efficiency. The hot metal surface is particularly subject to contamination by the paint's chemical residues.
Apparatus has also been devised to refocus the laser radiation reflected from the surface of the workpiece. Examples of these are described in U.S. Pat. No. 3,757,078 which issued on Sept. 4, 1973, to Conti et al; and U.S. Pat. No. 4,288,678 which issued on Sept. 8, 1981, to La Rocca. Since these devices attempt to refocus the reflected radiation, they produce a beam with a short depth of field and an irradiated area wherein the edges are poorly defined. A good beam edge sharpness is desirable in order not to heat treat a previously worked region, or a region where heat treating is not wanted.
Another problem which must be addressed is the non-uniformity across the laser beam caused by the spatial modulation of a multi-mode laser output as well as the tapered beam's edges. This results in partial vaporization of the surface in certain regions, while other regions are not raised above the temperature threshold for a martensitic transformation which will provide the desired working of the surface. A rapidly vibrating mirror has been used to integrate the laser power over a wide surface. Because of its complexity and the non-uniform exposure inherent to a sinusoidal scanning, this technique is now being replaced by optical integrating techniques. The best known is the mosaic of mirrors manufactured by the company SPAWR, which is now adopted by some major manufacturers of laser machining equipment, such as AVCO Everett. Such techniques are very effective in smoothing the multi-mode beam profile, but the sharpness of the beam's edges which they can produce is limited by diffraction.